WL#1658: Updatable Compressed Rows with MyISAM

Enable MyISAM to update tables which have been compressed with myisampack (each
record separately) (SPB DevConf 2004-02).
Additionally the compression system must be changable (Brian 2004-03-19). Either
by re-compressing all rows or by using it from now on and thus having
differently compressed rows in a table (Comment by Ingo 2004-03-22)?